Conference Paper
Room temperature accelerator structures for linear colliders
Stanford Linear Accelerator Center, Menlo Park, CA
DOI: 10.1109/PAC.2001.988264 Conference: Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001, Volume: 5 Source: IEEE Xplore
ABSTRACT
Early tests of short low group velocity and standing wave
structures indicated the viability of operating Xband linacs with
accelerating gradients in excess of 100 MeV/m. Conventional scaling of
traveling wave traveling wave linacs with frequency scales the cell
dimensions with λ. Because Q scales as λ^{1/2},
the length of the structures scale not linearly but as λ^{3/2 } in order to preserve the attenuation through each structure. For
the NLC we chose not to follow this scaling from the SLAC Sband linac
to its fourth harmonic at the Xband. We wanted to increase the length
of the structures to reduce the number of couplers and waveguide drives
which can be a significant part of the cost of a microwave linac.
Furthermore, scaling the iris size of the diskloaded structures gave
unacceptably high short range dipole wakefields. Consequently, we chose
to go up a factor of about 5 in average group velocity and length of the
structures, which increases the power fed to each structure by the same
factor and decreases the short range dipole wakes by a similar factor.
Unfortunately, these longer (1.8 m) structures have not performed nearly
as well in high gradient tests as the short structures. We believe we
have at least a partial understanding of the reason and will discuss it
below. We are now studying two types of short structures with large
apertures with moderately good efficiency including: 1) traveling wave
structures with the group velocity lowered by going to large phase
advance per period with bulges on the iris, 2) π mode standing wave
structures
structures indicated the viability of operating Xband linacs with
accelerating gradients in excess of 100 MeV/m. Conventional scaling of
traveling wave traveling wave linacs with frequency scales the cell
dimensions with λ. Because Q scales as λ^{1/2},
the length of the structures scale not linearly but as λ^{3/2 } in order to preserve the attenuation through each structure. For
the NLC we chose not to follow this scaling from the SLAC Sband linac
to its fourth harmonic at the Xband. We wanted to increase the length
of the structures to reduce the number of couplers and waveguide drives
which can be a significant part of the cost of a microwave linac.
Furthermore, scaling the iris size of the diskloaded structures gave
unacceptably high short range dipole wakefields. Consequently, we chose
to go up a factor of about 5 in average group velocity and length of the
structures, which increases the power fed to each structure by the same
factor and decreases the short range dipole wakes by a similar factor.
Unfortunately, these longer (1.8 m) structures have not performed nearly
as well in high gradient tests as the short structures. We believe we
have at least a partial understanding of the reason and will discuss it
below. We are now studying two types of short structures with large
apertures with moderately good efficiency including: 1) traveling wave
structures with the group velocity lowered by going to large phase
advance per period with bulges on the iris, 2) π mode standing wave
structures
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 "This coupling will resonantly amplify any betatron motion of the train, unless the transverse wakefield is reduced by about two orders of magnitude during the 1.4 ns between bunches. This difficult goal was met for the initial structure design by using a combination of cell detuning and damping [16] [17]. Detuning requires that each cell of the rf structure have a slightly different dipole frequency, such that the wakefields from the different cells have decohered significantly by the time the second bunch arrives. "
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ABSTRACT: The nextgeneration linear collider will require highpower microwave sources and accelerating systems vastly more challenging than its predecessor, the Stanford Linear Collider (SLC). Cost efficiency will demand high accelerating gradient to achieve beam energies five to ten times greater than in the SLC. Luminosity goals 10,000 times greater than the SLC demand efficient creation of the highest possible beam power without degradation of beam emittance. The past decade of R&D has demonstrated the feasibility of two technical approaches for building a 500GeV centerofmass system (cms) collider with attractive options for future upgrade. The TESLA R&D program offers the prospect of 1.3GHz superconducting rf (srf) linacs with 23.4 MV/m gradient that can be upgraded later to 35 MV/m gradient by doubling the number of klystrons and the cryoplant, to reach 800 GeV cms [1]. The Next Linear Collider (NLC) and Japanese Linear Collider (JLC) R&D programs offer the prospect of 11.4GHz roomtemperature linacs that can later be extended to 1 TeV by doubling the number of structures and klystrons, and to 1.5 TeV by additionally increasing gradient or length [24]. Both programs offer a 500GeV linear collider project start within the next few years (23 years for TESLA, 34 years for NLC) based on available technology validated by experiments at several complementary test facilities. Both offer their upgrades as a result of further progress in R&D that is already underway. 
 "This goal is not achieved in full yet and probably final QC and tuning of complete structure will still be a job for RF engineers. The structures we have produced so far are not the final NLC design, but they are various structures to support R&D on the breakdown problem [3]. So, we had to design the mechanical part of setups flexible enough to meet different structure designs. "
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ABSTRACT: As part of the Next Linear Collider (NLC) collaboration, the NLC structures group at Fermilab has started an R&D program to fabricate NLC accelerator structures in cooperation with commercial companies in order to prepare for mass production of RF structures. To build the Next Linear Collider, thousands accelerator structures containing a million cells are needed. Our primary goal is to explore the feasibility of making these structures in an industrial environment. On the other hand the structure mass production requires "industrialized" microwave quality control techniques to characterize these structures at different stages of production as efficiently as possible. We developed several automated setups based on different RF techniques that are mutually complementary address this problem.  [Show abstract] [Hide abstract]
ABSTRACT: Synchronous wakefield excitation and wave propagation along a dispersive slowwave structure is considered. An explicit form for wakefields is obtained for a single bunch in the second and third approximations of dispersion while taking into account the effect of substantial group velocity with respect to charge velocity. Generalized differential equations describing diffused fields induced by a beam current or generated by an external source are derived. Field excitation and propagation near the cutoff is considered including trapped modes in the stopband. This theory can be applied to the fields induced by single bunch and bunch train in Standing Wave and Traveling Wave devices operating near πmode, selfconsistent beam breakup simulations, RFgeneration, pulse propagation, and breakdown study in waveguides as well as some of new methods of acceleration in a dispersive medium.